The invention relates to gearshift-transmission linkage, and more particularly to a specially designed linkage for tilt-cab vehicles, enabling the linkage to remain fully connected while the cab is tilted forward on the frame for servicing.
Manufacturers of tilt-cab vehicles have had to address the problems that tilting imposes upon the control systems interfacing the cab with the chassis. Steering, shifter, clutch, and throttle linkages must all accommodate the articulation of the cab.
Shifter mechanisms generally pose the most complicated control function. Since a shift pattern is a two-dimensional array of gear positions, two channels of control are required. Cab-over-engine vehicles generally use a control unit that drives a single rod with a linear motion for one channel and a rotary motion for the other channel.
There have been two approaches to this problem. One has been the provision for upward swinging of the cab independent of the shifter, with the gearshift and associated mechanism remaining attached to the frame. By this arrangement, a frame mounted shift tower was provided to support the shifter control unit and a small "subfloor" attached to the control unit and positioned to seal against the underside of the cab floor. A rod and two universal joints connected the control unit directly to the transmission slave unit. When the cab was tilted forward, the cab floor simply lifted away from the subfloor, leaving the entire shifter system behind.
Subfloors have required careful alignment and presented problems in sealing against engine compartment noise, heat and fumes. Also, in order for the shift stick to pass through the opening in the floor when the cab is tilted, the shift knob usually could not be optimally located for driver comfort and ease of use. The large hole required in the cab floor both complicated and weakened the cab structure.
A second previous approach to the problem of tilt-cab vehicles utilized a break-away shift linkage. The shifter control unit was frame-mounted, as with the subfloor approach. The shift lever, however, was attached to the cab under this arrangement. Some means of disengagement of the shift lever from the control unit was provided to permit tilting of the cab.
A break-away shift linkage avoided many of the problems inherent in the subfloor approach, but was not a complete solution. Cab/frame alignment was critical and relative movement could result in large excursions of the shift lever. The break-away detail tended to introduce more lash in the system and reduced shifter "feel". It was usually vulnerable to contamination and tended to require frequent lubrication. As cabs have grown taller, shift towers have had to reach higher and many have had to be supported with extra tie rods and links to attain the necessary stability. All this has tended to add weight and restrict access to the left side of the engine.
U.S. Pat. Nos. 2,854,088 and 3,476,202 show several arrangements providing for pivoting of a shift linkage assembly with the pivot axis of shift linkage components located along the pivot axis of the cab. However, these systems were designed for tilt cabs quite different from those with which the present invention is concerned, and in each case the shift lever was positioned to extend directly down to the proximity of the cab's pivot axis. The type of linkage control motion was different.
It is among the objects of the invention to provide a tilt-cab shift linkage system for a truck of contemporary design, avoiding the problems associated with subfloors and break-away devices. Since reliability is of paramount importance, another object is to avoid complicated or unproven hardware. Push-pull cables, for example, are an obvious but failure prone solution.